Process for testing the suitability of a material for shaping without cutting and device for use in this process



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3,086,391 PROCESS FOR TESTING THE SUITABILETY 9F A MATERIAL FOR SHAPING WITHUUT CUTTING AND DEVECE FOR USE IN THES PRGJESS Karlheinz Sehrnitt-Thomas, Mainz (Rhine), and Fritz Fischer, Irlich, Germany, assignors to Stahland Wailwerke Rasselstein/Andernach AG, Neuwied (Rhine), Germany, a corporation of German Filed Mar. 6, 1.961, Ser. No. 93,556 Glaims priority, application Germany Mar. 8, 1969 7 Claims. (Q1. 73--87) The continuously increasing demands that are made in respect of the capability of sheet metal for being worked in deep drawing and pressing operations make it necessary for the manufacturer to examine by test all suitable ways of making the material suit the said demands.

In the case of the sheet metal intended for deep drawing or stamping or pressing the following features are regarded as particularly significant:

( 1) Analysis (carbon content, degree of purity and elements added to prevent ageing) (2.) Grain size and grain shape (3) Yield point (numerical value and, development of the yield point range).

In addition, however, the influence of the micro-surface on the deep drawing procedure has been recognised to an increasing extent. In unfavourable cases this influence can be so great that with other optimal properties prescribed qualified deep drawing is hardly possible.

Up to the present time attempts have been made to relate a picture of the surface profile obtained by feeling and recording and measurements obtained therefrom (roughness value) to the subsequent course of events in deep drawing. In practice this led to conflicting opinions and to results which were variable and could not be reproduced. As explanation of this state of affairs is to be found in an analytical consideration of the friction effects which occur in deep drawing.

summarising, the mechanism of the friction is as follows:

(1) The two bodies that are in contact in deep drawing penetrate so far into one another under suitable pressure that suitably loaded points of the two surface profiles provide static equilibrium.

(2) Upon the subsequent application of a horizontal force they are finally shifted relatively to one another. The forces necessary for this are composed of shearing in particular,

forces which are occasioned by the mutual shearing action of contacting and inter-engaging points, and of cutting forces which arise from the drawing of one surface through the other. With this there is associated a further interpenetration of the two contacting bodies.

Having regard to these considerations, the inadequacy of the above-described known measuring processes for use in deep drawing will be clear. Exhaustive tests on which the present invention is based have shown that for a material which is best suited for deep drawing a quite definite surface characteristic is necessary. This surface characteristic is determined not only by the geometric shape of the surface profile but also by the technological properties thereof, i.e., a characteristic whichhas meaning must always include these two components. The characteristic of a surface which is best suited for shaping without cutting, in particular by deep drawing, is defined by the following features:

(1) The geometrical form and composition of the surface profile is divided into as large a number as possible of uniformly distributed slender peaks in such a manner that as the distance of the profile section from the envelope increases the increase in the load-bearing fraction is relatively small.

(2) The material of the surface profile must be so well capable of deformation, i.c., it must have suitably low values of yield point, strength and hardness, that at the pressure employed the tool penetrates relatively deeply into the surface profile, this pressure being taken up by a plurality of uniformly divided peaks which are flattened by the deformation, and the forces necessary for drawing the two surface profiles which are under pressure through one another are relatively small.

This surface characteristic shows that the degree of penetration of the tool into the surface profile in accordance with the pressure and also the force required to displace the material under the tool under pressure are of decisive importance for the deep drawing procedure.

Starting with this knowledge, the testing process according to the invention consists in that a test piece with an effective surface corresponding preferably to the surface of the tool is loaded from Zero or a very low starting pressure up to a maximum adjustable pressure, and the depth of penetration of the two mutually acting surfaces is measured in relation to the pressure. In accordance with the invention, after the depth of penetration under the set pressure has been measured this pressure is maintained and there is exerted on the test piece an increasing force which is at right angles to the pressure and which initiates the displacement of the test piece relative to the eifective pressure surface. The further depth of penetration of the mutually acting surfaces is then measured as well as the force required for initiating the displacement. These measured values, or advantageously the curves obtained from these values, can be compared with the result obtained by drawing. In this way, therefore, it is possible, with the new testing process, to determine in advance and very exactly the behaviour of a material intended for deformation without cutting, in particular its behaviour in deep drawing.

The testing process according to the invention will now be described with reference to the accompany drawings, in which FIG. 1 shows various surface profiles of test pieces FIG. =12 shows the relationship of the drawing results on the load-bearing fraction of the surface profile shown in FIG. 1,

FIG. 3 is a diagrammatic representation of test apparatus according to the invention for recording surface characteristics, and

FIGS. 4 and 5 show with this test into account.

In FIG. 1 there are illustrated various surface profiles 1 to 7 with the associated roughness values. These relate to sheet metal plates for deep drawing with substantially similar technological properties, with which deep drawing tests were carried out (production of a difiicult deep surface characteristics obtained apparatus taking the drawing procedure s,ose,391

drawn part in one pull). The drawing results in relation to load-bearing fraction are illustrated in FIG. 2.

FIGS. 1 and 2. show how the drawing results depend on the geometry of the surface, in particular on the increase in the load-bearing fraction with increasing distance between the profile section and the envelope. If we assume that the technological properties of the profiles compared are the same, as has to be assumed for the test material, then a low increase in the load-bearing fraction with depth of the profile section corresponds to a soft characteristic and a large increase in the load-bearing fraction with increase in distance between the profile section and the envelope corresponds to a hard characteristic. With these assumptions, for the same pressure a tool penetrates further in the surfaces to 7 than into the surfaces 1 to 3 until the load-bearing fraction necessary for taking up a predetermined pressure is reached. In FIGS. 1 and 2 it can be seen how a surface brought into contact with the profiles 1 to 3 immediately encounters large load-bearing surface elements, in a correspondingly irregular distribution over the whole surface. At a predetermined pressure there is in this case only slight penetration since the load-bearing surface elements already suffice, after this slight depth of penetration, to take up the pressure. The profiles 5 to 7 in FIGS. 1 and 2, on the other hand, require deeper penetration of the surfaces in contact, before the load-bearing surface elements suffice to take up the pressure. The shape of these surface profiles 5 to 7 accordingly results in surface contact at elements which are comparatively uniformly distributed over the whole surface. Due to such a uniform distribution of the surface element taking up the pressure, likewise the points of application of the forces which oppose displacement of the bodies in contact are distributed uniformly over the whole surface. In contrast thereto, the frictional forces in the case of the surfaces 1 to 3 can act only at the relatively large, spaced, surface elements non-uniformly distributed over the surface. This results in unfavourable behaviour of the material in deep drawing. Either fold formation occurs due to the non-uniform surface contact or the increase in the pressure of application of the tool to produce uniform application thereof to the surface of the metal sheet must be such that the deforming forces necessary for drawing result in tearing of the material.

If now, instead of the profiles previously considered whose geometrical properties are different, we consider a case in which the technological properties of the profiles are different, then a comparative consideration of the geometry of the profiles no longer suffices to enable one to estimate their behaviour under the forces arising in deep drawing. For example, if the plasticity of the profiles of the surfaces 1 to 3 is sufficient, a surface pressed into contact with these profiles will penetrate relatively deeply and thereby encounter a sufliciently uniform abutment surface, whereas assuming that in the case of the surfaces 5 to 7 the profile material is very strongly compressed there will be only slight penetration of a surface pressed into contact with these profiles. These considerations lead to the conception of a surface characteristic which consists of two components, viz. geometry and technological properties of the profile, for the purpose of clearly estimating the behaviour of a surface in relation to deep drawing.

FIG. 3 shows diagrammatically an apparatus suitable for carrying the test process according to the invention into practice and enabling the surface characteristic to be recorded. The apparatus consists of a movable die 10 which is loaded with adjustable presure and which has an effective pressure surface 11, and a stationary counter die 12 the effective pressure surface 13 of which is preferably of same size as the surface 11 of the die 10. The co-acting pressure surfaces 11 and 13, which are preferably exchangeable, correspond as regards material and treatment to the tool used for the deforming operation, which in deep drawing is the punch. If desired, however, these co-acting pressure surfaces may be completely fiat and extremely hard (e.g. they may be made from hard metal) to enable absolute values to be obtained when using the testing apparatus according to the invention. These co-acting pressure surfaces 11 and 13 are of such dimensions that they penetrate only into the surface material of the test piece but not into the underlying material.

The test piece 14, e.g., a piece of sheet metal for deep drawing, is placed between the die 10 and counter-die 12, the die 10 then acting on the test piece. In the arrangement diagrammatically illustrated it is possible to adjust the loading of the die 10 exactly by adjusting a loading weight 14 relatively to a scale 17 on a beam 16 pivotal abut the axis 15. The test piece is therefore loaded from zero or a predetermined initial load up to the set value. At the commencement of the testing operation the initial loading can be set to a definite value so as to ensure that the test piece lies firmly between the dies 10 and 12. The very small distance that the die 10 has to move until the set loading is reached is then measured by means of the apparatus shown in relation to the load pressure. For this purpose, in the diagrammatic embodiment, there is provided a mirror 19 which is pivotable about the axis 18 and which has a knife-edge 26 which bears on the stationary die 12. The axis of rotation 18 of the mirror is journalled in a bearing block 21 which is rigidly connected to the die 10. The light beam 24 from a light source 22 is projected by the mirror onto a scale 23 from which the depth of penetration can be read off. The light beam impinging on the scale at the commencement of the measuring operation (zero or initial loading) is shown at 24a and the light beam impinging on the scale at the end of the measuring operation is shown at 24b.

When testing the suitability of the material for deep drawing not only is the depth of penetration s measured in relation to the pressure P of the die 10 caused by the adjusted weight 14, but also the behaviour of the test piece 14 is measured when this is caused to slide between the dies 10 and 12 by a horizontal force H while still under pressure. As shown in the drawing this force H acts via a scale 25 and a tension spring 26 on a mounting block 27 which supports the test piece 14. The value of the force H at any time can be read off from the scale 25 by means of a pointer 28 rigidly connected to the block 27. When under the load pressure P the corresponding abutment points 8 (FIG. 1) have produced static equilibrium corresponding to the pressure and a certain degree of penetration s has been reached. The horizontal force H applied to the test piece then is increased until the test piece moves between the dies 10 and 12. At the instant at which this movement takes place, according to theory (co-operation of pressure and shearing force) further pentration of the die 10 into the surface profile of the test piece occurs. This increase in the degree of penetration is measured on the scale 23 and similarly the corresponding value of the displacing force H is measured on the scale 25.

In accordance with the invention, the degree of penetration s is measured in relation to the increase in pressure P, or with the pressure maintained, in relation to the increasing displacing force H and is indicated by a curve which is then related to the result obtained by drawing. Such qualitative curves, which correspond to the load on operation of the apparatus shown in FIG. 3, are shown in FIGS. 4 and 5. The ordinate 29 relates to the degree of penetration s. The abscissa 30 to the right of the ordinate represents the pressures P which are suitably selected and, as can be seen from the curves, approach the selected maximum load proceeding from right to left towards the ordinate. The left hand part of the abscissa represents the displacing forces H which are required to bring about displacement of the test piece when the prescribed loading pressure has been reached. The degree of penetration associated with the two forces P and H can then be read off in full from the curves. The ordinate shows the degree of penetration in ,u.

In a suitable series of tests which relate the curves to the drawing results, cases are considered in which there is drawing with tearing, in which there is drawing with folding and in which the drawing operation is good. These cases are shown in FIGS. 4 and 5. I will be seen from the drawing that if the degree of penetration s is too small fold formation will occur, or the degree of penetration must be of at least the value s to enable good drawing without fold formation to be effected. On the other hand tearing of the drawn material occurs if the displacing force H acting on the test piece is too large, or if the displacing force H1 is exceeded.

After these definite regions for folding, tearing and good drawing have been obtained, a simple measurement with the testing process or apparatus according to the invention will give information as to the phenomenon associated with the tested surface in deep drawing, i.e., the behaviour of the test piece when deformed without cutting, particularly its behaviour in deep drawing, can be definitely determined in advance.

In FIG. 4 the curves 1, 2, 4 and 5, 6, 7 represent different cases of the behaviour of the surface corresponding approximately to the surface profiles 1, 2, 4 and 5, 6, 7 of FIG. 1, it being assumed that the materials have the same technological properties particularly in the surface profile.

The curve 1, 2 corresponds to a smooth surface which yields only slightly under pressure and therefore allows a slight degree of penetration which does not increase appreciably even when the displacing force H is applied. This characteristic curve means that satisfactory drawing is impossible. An increase in pressure which would produce sufficient uniformity of contact between the tool and the surface of the material would have to be so large that tearing would occur during deep drawing. This is shown in FIG. by the curve 1", 2" for a pressure increase to P4.

Curve 4 relates to a material with an average characteristic (profile 4, FIG. 1).- The combined action of the pressure and shearing forces causes a penetration of the surface of the die into the surface of the test piece which is sufficient to enable deep drawing to be effected with slight fold formation. An increase in the degree of penetration s and therewith greater uniformity of application, which is necessary for complete suppression of fold formation, is obtained with an increase in the pressure of the punch or in the test pressure.

FIG. 5 shows how such a pressure increase to P3 causes the curve 4 to enter the region in which good drawing is possible.

Finally, there is shown by the course of the curves 5, 6, 7 the surface characteristic of a material which as regards its surface can be regarded as very suitable for deep drawing.

Curves 5a, 6a, 7a show the behaviour of a very soft material of low yield point and low strength, particularly in the surfaceprofile. Under the initial loading P1 there is indeed a high degree of penetration s, as in curves 5, 6, 7. The effective surface will, however, be moved through the mass of the underlying material under displacing forces H which are above the tensile strength of the material, so that tearing occurs. FIG. 5 shows how under some circumstances a reduction in the punch pressure to P2 can still enable good deep drawing to be effected in this case also, as is illustrated in curves 5a, 6a, 7a.

The possibilities and curves shown and discussed naturally represent only individual cases from a large number of possibilities which, however, are all explained or given meaning in the described measurement and represen'tation of the surface characteristic by the testing process according to the invention.

The process according to the invention is intended not only for testing the suitability of material for deep drawing, but may in some cases be used also for testing the suitability of material for another process of shaping without cutting, e.g., stamping. The new testing process ismoreover not limited to metals but can if desired also be used for testing the surface characteristic of synthetic materials.

If desired the new process can also be used when the material is subjected in-the shaping. process only to pressure and not to additional displacing forces. Thus the new testing process can also be used for measuring the hardness of surface profiles e.g., in the technique of bearings (sliding and roller bearings). In this case measurement of the degree of penetration under pressure alone is sufficient.

We claim:

1. A method of testing sheet material which comprises:

pressing the sheet material between two smooth surfaces of large area with a first force which is directed substantially perpendicularly to said sheet material to thereby effect flattening of the surface profile of said sheet material;

measuring the distance said Surface profile is flattened in response to said first force;

applying a second force directed substantially perpendicularly to said first force. to urge said sheet material to move laterally with respect to said surfaces and simultaneously urging said surfaces toward each other and into gripping engagement with the sheet material to oppose lateral movement thereof; and

measuring the further flattening of said surface profile effected when said sheet material moves laterally with respect to said surfaces.

2. A method of testing sheet material which comprises:

pressing the sheet material between two parallel,

planar, pressure surfaces with a first force of selectable value so that only the surface profile of the sheet material is penetrated;

measuring the distance said surface profile is penetrated in response to said first force;

applying a progressively increasing second force on said sheet material urging same to move in a direction parallel with said pressure surfaces while simultaneously maintaining said first force on said sheet material; and

measuring the further penetration of the surface profile of the sheet material elfected by said pressure surfaces when said sheet material moves in said direction with respect to said pressure surfaces.

3. A method according to claim 2 including the further step of measuring the value of said second force when said sheet material begins to move in said direction.

4. A method according to claim 3 including the step of continuing to increase the value of said second force and measuring the amount of penetration of the surface profile of the sheet material effected by selected values of said second force to provide data whereby a graph may be drawn to show the relation between the amount of penetration and the value of the second force.

5. A method according to claim 2 including the step of adjusting the first force through a series of selectable values and measuring the amount of penetration of the surface profile of the sheet material and said selectable values of said first force to provide data whereby a graph may be drawn to show the relation between the amount of penetration and the value of the first force.

6. Apparatus for testing sheet material comprising:

a pair of parallel, planar, pressure surfaces which are movable toward and away from each other and which are adapted to receive the sheet material therebetween;

means for urging said surfaces toward each other under a selectable pressure;

means for measuring the amount of movement of said pressure surfaces toward each other whereby the amount of penetration of the surface profile of the sheet material etfected by said surf-aces can be measured;

means for gripping the sheet material located between the pressure surfaces and urging the sheet material to move with respect to said pressure surfaces in a direction parallel therewith and;

means for measuring the force exerted by said gripping means on the sheet material.

7. Apparatus as claimed in claim 6 in which the pressure surfaces correspond in hardness and surface finish to the tools intended to be used for shaping the sheet material.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A METHOD OF TESTING SHEET MATERIAL WHICH COMPRISES: PRESSING THE SHEET MATERIAL BETWEEN TWO SMOOTH SURFACES OF LARGE AREA WITH A FIRST FORCE WHICH IS DIRECTED SUBSTANTIALLY PERPENDICULARLY TO SAID SHEET MATERIAL TO THEREBY EFFECT FLATTENING OF THE SURFACE PROFILE OF SAID SHEET MATERIAL; MEASURING THE DISTANCE SAID SURFACE PROFILE IS FLATTENED IN RESPONSE TO SAID FIRST FORCE; APPLYING A SECOND FORCE DIRECTED SUBSTANTIALLY PERPENDICULARLY TO SAID FIRST FORCE TO URGE SAID SHEET MATERIAL TO MOVE LATERALLY WITH RESPECT TO SAID SURFACES AND SIMULTANEOUSLY URGING SAID SURFACES TOWARD EACH OTHER AND INTO GRIPPING ENGAGEMENT WITH THE SHEET MATERIAL TO OPPOSE LATERAL MOVEMENT THEREOF; AND MEASURING THE FURTHER FLATTENING OF SAID SURFACE PROFILE EFFECTED WHEN SAID SHEET MATERIAL MOVES LATERALLY WITH RESPECT TO SAID SURFACES. 